Power supply unit and related lighting system

ABSTRACT

The Power Supply Unit comprises an output providing electrical power between a positive power supply line and a common ground line and a communication line. An adjustable current generator responsive to an internal measurement signal generates an output current at the output, and a voltage source is coupled to the communication line. A current measurement unit measures a current through the communication line and generates the internal measurement signal depending on the measured current through the communication line. Specifically, the Power Supply Unit is configured to determine the voltage drop on the common ground line by applying via the voltage source two different voltages to the communication line.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of copending application Ser. No. 14/409,479filed on Dec. 19, 2014 which is a national stage entry according to 35U.S.C. §371 of PCT application No.: PCT/IB2013/055106 filed on Jun. 21,2013, which claims priority from Italian Application No.: TO2012A000558filed on Jun. 25, 2012, and is incorporated herein by reference in itsentirety.

TECHNICAL FIELD

The present disclosure relates to the field of Solid State Lighting, anddescribes an interface for a Light Engine Module to its Power SupplyUnit and the Light Engine Module respective the Power supply unit. Thepresent disclosure generally relates to a Power Supply Unit for drivingone or more Light Engine Modules, in particular Light Engine Moduleswith light-emitting diode (LED) light sources, and a lighting unitincluding a Power Supply Unit and at least one Light Engine Module. Moreparticularly, various inventive methods and apparatus disclosed hereinrelate to a self-adjusting Power Supply Unit for driving one or moreLight Engine Modules with light-emitting diode (LED) light sources, andan LED− based lighting unit including a self-adjusting Power Supply Unitand at least one Light Engine Module.

BACKGROUND

Illumination devices based on semiconductor light sources, such aslight-emitting diodes (LEDs), offer a viable alternative to traditionalfluorescent, HID, and incandescent lamps. Functional advantages andbenefits of LEDs include high energy conversion and optical efficiency,longer expected lifetime, lower operating costs, and many others.

In some applications, an LED-based lighting unit may include a PowerSupply Unit which supplies an LED driving current to a plurality ofLight Engine Modules, each including one or more LEDs. For example, anLight Engine Module may include a circuit board (e.g., a printed circuitboard) having one or more LEDs mounted thereon. Such circuit boards maybe plugged into slots in a lighting fixture, or a motherboard, on whichthe Power Supply Unit may be provided.

In various applications and installations, an LED-based lighting unitmay include different numbers of LEDs and/or Light Engine Modules. Forexample, the number of LEDs and Light Engine Modules may be changeddepending on the light output requirements, e.g. lumens, for aparticular installation.

In general, the magnitude or level of the LED driving current output bya Power Supply Unit will need to be changed according to the number ofLEDs and Light Engine Modules to which it is connected and which itdrives. This means that if a single Power Supply Unit is going to beemployed in a variety of LED-based lighting units with different numbersof LEDs and/or Light Engine Modules, then the Power Supply Unit willhave to include a means or provision for adjusting the LED drivingcurrent to match the current driving requirements for the differentLight Engine Modules according to the different numbers of light sourcesthat they include. Meanwhile, the number of LEDs and Light EngineModules to be included in a particular LED-based lighting unit isdetermined at the time of manufacturing that LED lighting unit. Thus, ifthe same Power Supply Unit is to be employed in a variety of LEDlighting units with different numbers of Light Engine Modules, then thepower supply unit would have to be programmed at the time ofmanufacturing for each different LED lighting unit so that its outputLED driving current is appropriate for the particular number of LightEngine Modules that are included in that LED lighting unit.

This problem has been addressed by means of interfacing between PowerSupply Unit and Light Engine Module.

Interfacing means that the Light Engine Module provides the Power SupplyUnit with some information, regarding its needed current to fulfil fluxspecification and/or its working temperature, in order to reduce thesupplied current level when a certain limit is exceeded. There areseveral ways in the Art to interchange this information between theLight Engine Module and the Power Supply Unit. Buses can be used tointerchange such information. Known in the art are analog buses like the0.10V bus or digital buses like the DALI (Digital Addressable LightInterface) bus. Also known in the Art are simple Resistor networks thatcan be measured by the Power Supply Unit and tell the Power Supply Unitthe current requirements of the Light Engine Modules. DE 100 51 528discloses such an interface where a specific Resistor is connectedbetween a third wire and the negative supply line. If several LightEngine Modules are connected to one Power Supply Unit, the resistors areconnected in parallel or serial, so a sum signal is given into the PowerSupply Unit to define the current requirements. The German patentapplication 102011087658.8 discloses also resistors to define thecurrent requirement of each Light Engine Module.

The bus solutions have the disadvantage of two extra wires needed. Theresistor solutions only need one extra wire, but the evaluation of theresistor network and the current adjustment can be very complex.

Since complete Power Supply Unit and Light Engine Module systems haveappeared on the market, different companies have tried to fix a way tomake the two parts communicate; also some digital protocols have beenused for the more complex and high-end systems, but this lattertechnique is out of the present disclosure's background, and have to beconsidered apart.

For instance, the company OSRAM has already proposed a three extra-wireinterface, able to supply also power to an active Light Engine Moduleonboard circuitry which provides thermal derating. In this interfacetype a Light Engine Module onboard resistor forms a divider with a PowerSupply Unit pull-up resistor, in order to develop a voltage which setsthe Power Supply Unit output current. An operational amplifier on theLight Engine Module then starts to limit this voltage (so reduces thecurrent) when the module overheats.

The company Philips has proposed a different extra-three wire interface,where one wire is connected to the current setting resistor, whileanother one is connected to a temperature sensing resistor, and thederating is done by the Power Supply Unit itself, not involving anyactive part on the Light Engine Module.

Both interfaces include a third extra wire for the common signal groundreturn, and use a voltage developed by the Light Engine Module resistorto set the current, in such a way that the greater voltage causes thegreater output current.

Recently, the company OSRAM has come out with a slightly differentinterface, that actually is a 0.10V one customized with a precisecurrent source in the Power Supply Unit to enable the Light EngineModule to use just a resistor to set the current.

Now a new request rises from the market, i.e. the capability ofparalleling different modules to be supplied by the same Power SupplyUnit. Obviously the Power Supply Unit's outputted current must be thesum of each Light Engine Module nominal value, and the thermal deratingcapability must be kept even for a multiple Light Engine Modulearrangement.

As well, the market is asking for a cost cut, actually pointing to awire number reduction. Bus-based interfaces normally need 4 wires, twofor the power supply of the Light Engine Modules and two for the bus.

So a couple of new features to satisfy the needs have been postulated:

-   -   Multiple modules must be allowed to be connected in parallel        using the same interface (of course the different modules are        supposed to be identical, or at least to have the same string        voltage).    -   The setting interface must have a reduced number of wires, and        must be as simple as possible in order to reduce costs,        especially at the Light Engine Module side.

All the known interfaces proposed up to now are not able to supportmultiple Light Engine Module connections, a new interface is proposed inorder to fulfill all the newest requirements.

SUMMARY

In various embodiments, the way to provide the current requirementinformation is analog, i.e. by the magnitude of some electricalparameters directly set by the onboard circuitry of the Light EngineModule: these parameters are then recognized by the Power Supply Unitwhich adjusts its output current as demanded.

Hereafter both a concept and a possible implementation of a “one wire”analog interface are proposed, with “one wire” meaning that only oneextra wire is need besides the two power wires.

Various embodiments are directed to a Light Engine Module including:

-   -   a plurality of series connected LEDs    -   a positive power supply line    -   a common ground line    -   a communication line where signals on the communication line are        measured against the common ground line,    -   a current set resistor with its value proportional to the        current demand of the Light Engine Module,    -   an amplifier responsive to a temperature sensitive input        current,    -   a temperature sensitive resistor responsible for the temperature        sensitive input current.

This Light Engine Module is easy and cheap to build and can handledifferent current demands and thermal derating over only one extra line.

The conductance of the current set resistor is preferably directproportional to the current demand of said Light Engine Module. Thiseases the processing of the measured signal.

Additionally, the Light Engine Module is preferentially including athreshold set resistor responsible for a threshold level for thermalderating. This has the advantage of thermal derating only taking placeat higher temperatures so no light will be lost in nominal operation.

The Light Engine Module may include also:

-   -   a voltage source with the negative output connected to common        ground,    -   a series connection of the temperature sensitive resistor and        the threshold set resistor connected in parallel to the voltage        source,    -   a transistor where the base is connected to the node between the        temperature sensitive resistor and the threshold set resistor,        the collector is connected to the node between the positive        output of the voltage source and the temperature sensitive        resistor, the emitter is connected to the communication line via        a emitter resistor.

This current source is capable of conducting a duration of the suppliedcurrent to substantially zero to protect the Light Engine Module.

The Light Engine Module may include in an alternative solution:

-   -   a voltage source with the negative output connected to common        ground,    -   a series connection of the temperature sensitive resistor and        the threshold set resistor connected in parallel to the voltage        source,    -   a transistor where the base is connected to the node between the        temperature sensitive resistor and the threshold set resistor,        the emitter is connected via a emitter resistor to the node        between the positive output of the voltage source and the        temperature sensitive resistor, the collector is connected to        the communication line.

In another embodiment the voltage source is derived from the two powersupply lines.

In a further embodiment the voltage source is connected with commonground and tapped between a portion of the plurality of series connectedLEDs.

Various embodiments are also directed to a Power Supply Unit including:

-   -   an output providing electrical power between a positive power        supply line and a common ground line,    -   a communication line where signals on the communication line are        measured against the common ground line,    -   an adjustable current generator responsive to an internal        measurement signal generating an output current at the output,    -   a voltage source coupled to the communication line,    -   a current measurement unit generating the internal measurement        signal.

This allows an easy and cheap Power Supply Unit with adequate precision.

In a first embodiment the current measurement unit includes a currentmeasurement resistor and an operational amplifier.

In a second embodiment the Power Supply Unit has the followingattributes:

-   -   the voltage source is connected between common Ground and the        positive input of the operational amplifier,    -   the current measurement resistor is connected between the        negative input of the operational amplifier and the output of        the operational amplifier,    -   the communication line is connected to the negative input of the        operational amplifier,    -   the output of the operational amplifier outputs the internal        measurement signal.

This enhances the precision and leads to a cheap and precise powersource.

In a further embodiment the voltage source is adjusted in a way, thatevery single Light Engine Module can decrease the supplied current tosubstantially zero. This protects the Light Engine Modules.

In a further embodiment the Power Supply Unit conducts linearcharacteristic between the internal measurement signal and the outputcurrent of the adjustable current generator. This leads also to an easyand cheap measurement unit.

In a still further embodiment the linear characteristic takes place onlyover a portion of the total range of the internal measurement signal.This generates a threshold for the temperature derating.

In a still further embodiment the beginning of the portion is defined byan offset voltage of the internal measurement signal, and below thisoffset voltage no current will be provided by the adjustable currentgenerator. This also is good for the temperature degradation.

In another embodiment the offset voltage is identical to the value ofthe voltage source. This leads to an easy implementation.

In another further embodiment the value of the current measurementresistor is defined by the maximum current output of the adjustablecurrent generator. This helps to scale the Power Supply Unit in regardto the current capabilities.

Various embodiments are also conducted to a lighting system, including:

-   -   a Power Supply Unit;    -   at least one Light Engine Module;

wherein the Power Supply Unit and the Light Engine Module have aninterface as described above, providing information to the Power SupplyUnit on the current demand of the connected Light Engine Modules.

In a first embodiment of the lighting system the Power Supply Unitincludes an identification voltage source supplying a Light EngineModule identification voltage and measuring the correspondingidentification current. This leads to an easy and cheap interfacebetween the Power Supply Unit and the Light Engine Module.

In a second embodiment all Light Engine Modules are connected inparallel. This leads to a very simple handling for the customer.

As anticipated, the features of this analog interface are:

-   -   To tell the Power Supply Unit which is the nominal supply        current of the Light Engine Module.    -   To reduce the supplied current e.g. when the temperature is        higher than expected. This is sometime referred as “thermal        derating”, and it is an event caused by unpredictable reasons.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. The drawings are not necessarilyto scale; emphasis instead generally being placed upon illustrating theprinciples of the disclosed embodiments. In the following description,various embodiments described with reference to the following drawingsin which:

FIG. 1 shows the paralleling concept of current set resistors;

FIG. 2 shows a simple solution for thermal derating;

FIG. 3 shows the complete concept of the present disclosure with thethermal derating unit TDU;

FIG. 3 shows the complete concept of the present disclosure with thethermal derating unit TDU;

FIGS. 4A and 4B show a very simple TDU implementation;

FIG. 5 shows a simple system implementation. V_(out) is the internalvoltage representing the output current;

FIG. 6 shows a simulation graph of the circuit of FIG. 5;

FIG. 7 shows a schematic circuit of how to model the cable voltage dropdue to LED current;

FIG. 8 shows a characteristic of the Current Generator; and

FIG. 9 shows a characteristic of the Current Measurement Unit.

DETAILED DESCRIPTION

The following detailed description refers to the accompanying drawingthat show, by way of illustration, specific details and embodiments inwhich the disclosure may be practiced.

In the following, several embodiments of the inventive concept will bedescribed. The inventive concept always deals with a three wireinterface, where several Light Engine Modules can be connected inparallel to a Power Supply Unit and the current requirements of everyLight Engine Module match.

First Embodiment: Analog Circuit

The basic idea of having a resistor to set the current has been kept,but the inventive concept of using it is different. FIG. 1 shows thegeneral paralleling concept of current set resistors. Three Light EngineModules LEM connected to a Power Supply Unit PSU are shown. Theconnection consists of three lines: A supply line LED+, a common groundline LED− and a communication line CL. Each Light Engine Module containsat least one LED string. The LED string consists of a plurality of LEDs.A plurality in the light of the disclosure means that there are at leastthree LEDs connected in series. Each Light Engine Module also contains acurrent set Resistor Rset. The current set Resistors are connectedbetween the common ground line LED− and the communication line CL. Thisleads to a parallel connection of each current set Resistor Rset1,Rset2, Rsetm, so the Power Supply Unit PSU measures the equivalentresistance of that parallel connection. The concept is to have the PowerSupply Unit PSU reading not a voltage as in the related art, but acurrent representative for the resistance value. Then an inverse law isapplied to the resistance value to set the Power Supply Unit's outputcurrent. The law is as follows:

Kv has the dimension of a voltage.

$I_{Output} = \frac{Kv}{R_{Set}}$

By doing so, the Power Supply Unit's output current is inverselyproportional to the Light Engine Module current set resistor value Rset,i.e. the smaller the resistance, the higher the output current of thePower Supply Unit PSU.

This intrinsically satisfies the requirement of having a final currentequal to the sum of each single Light Engine Module one, according tothe well-known Ohm's law.

FIG. 2 shows a concept schematic of an interface with a thermal deratingcapability. This adds a very simple thermal derating by putting a PTCelement in series with Rset. As the temperature of the Light EngineModule LEM rises, the value of the PTC also rises leading to a smallercurrent for that module. The disadvantage of such an arrangement is thatit won't be adequate for a multiple Light Engine Module connection,because a single PTC action would take away from the sum of the parallelconnected resistors Rset only that member's contribute, and this couldbe not enough to reduce the suffering Light Engine Module's temperatureenough.

Anyhow this solution could be kept for very low-cost applications, whena partial current reduction in the event of overheating is stillacceptable.

Furthermore, a simple temperature element in series with the currentsetting resistor has the disadvantage of continuously derating thecurrent, without having a precise starting point for the derating itself(even if some PTC elements have a very steep behavior around the triggertemperature). So the “nominal” current setting would be corrupted by a“parasitic” effect of the derating element.

FIG. 3 shows the inventive concept of an interface with a thermalderating unit TDU.

The concept relies on a different approach, by adding an extra currentgenerator TDU onboard the Light Engine Module. This current generator istemperature controlled by a sensing element, and takes power directlyfrom the Light Engine Module's power line, in order to avoid extra wiresfor the interface. The current generator includes a temperaturesensitive resistor generating an input current and an amplifieramplifying that input current to the generated current I_(TDU). Thegenerator is arranged with a threshold which inhibits any currentinjection until a certain over-temperature of the Light Engine Module isachieved. Then the slope of current versus temperature (gain of I_(TDU))is high enough for the system to try to stabilize the max workingtemperature of the Light Engine Module, but not so to triggerinstabilities due to heat transmission time lags. The current generatoris able to override completely the signal generated by the paralleledresistors Rset: in such a way it can safely protect the whole system andespecially its own Light Engine Module even in case of multiple LightEngine Module connection together with a very concentrated overheating.

With the temperature dependent current generator a new problem arises.It is necessary to measure Rset independent of the actual temperature ofthe module and therefore independent of the provided current of thecurrent generator. The way to measure Rset out must be fixed in order tomake the action of the current generator predictable.

In various embodiments, the disclosure uses a fixed voltage generator Vkto measure the resistance value, by putting this voltage across theresistor Rset (or their parallel) and then reading the current flowingthrough it. This in turn makes the current generator TDU directlyinteracting with the current fixed by Vk on Rset, resolving the finalbehavioral law.

FIG. 4A shows a first embodiment of the Light Engine Module providingthe inventive interface, with just one bipolar transistor, an NTCelement and a couple of added resistors.

The circuit contains a voltage source V1, which is derived from thesupply line LED+ of the LED module. LEDs have a quite stable fluxvoltage, so this can serve as a voltage source “good enough”. Dependentof the supply voltage needed for TDU, the voltage source V1, alwaysconnected to common ground LED−, can be tapped between a portion of theplurality of series connected LEDs. This means, the voltage V1 can beadjusted in a way that it represents a multiple value of a single LEDflux voltage. In parallel to this voltage V1 there is a seriesconnection of the NTC and a threshold resistor Rthr. The base of a NPNBipolar Junction Transistor (BJT) Q1 is connected to the node betweenthe NTC and Rthr. The collector of Q1 is connected to the voltage V1.The Emitter of Q1 is coupled to the communication line via an emitterresistor Rtg. All these components of FIG. 4a described above areforming the thermal derating unit TDU.

The current set Resistor Rset is connected between the rail-wisepositioned CL and common ground LED− lines of the power supply.

In this circuit the potential of Q1's emitter is referred to a forcedvoltage (by definition Vk) in the Power Supply Unit PSU that realizesthe threshold below which no current I_(TDU) is injected into the CLline. When the temperature rises, the NTC starts to raise the basepotential, until moving Q1 into the active region. Now the emitterresistor Rtg sets the gain of the circuit TDU, and fixes the slope ofthe injected current I_(TDU) versus temperature.

The resistor Rthr, together with the NTC at the temperature triggerspecified for the TDU, sets the thermal derating starting point inrelation to the voltages V1 and Vk.

A further advantage of this arrangement is the good linearity of thecurrent I_(TDU) versus temperature achievable.

One of the most interesting advantages of the disclosure, besides theeasiness of the implementation on the Light Engine Module side, is itscapability to be used in different quality grade systems, by adjustingthe wanted accuracy and features only by scaling the Power Supply Unitinterface's circuitry complexity. In other words, it's possible to buildthe reading interface on the Power Supply Unit side according to therequested accuracy and/or extended features needed.

FIG. 4B shows a second embodiment of the Light Engine Module LEMinterface with a dual implementation. Here a PNP-Type Transistor Q2 isused together with a PTC. A PTC is a temperature sensitive resistor witha positive temperature coefficient. The voltage V1 is derived fromeither the whole number of series connected LEDs or a portion of theseries connected LEDs. In contrary to the embodiment of FIG. 4A, thecollector of Q2 is providing the current source characteristic producingthe current I_(TDU), and is connected to CL. Thus the temperaturederating threshold is not depending on Vk but only on V1 and the valuesof the voltage divider formed by the temperature sensitive resistor PTCand the threshold resistor Rthr.

Thus, in the embodiments discloses in the forgoing the Light EngineModule LEM includes a set resistor Rset to specify the nominal currentand a thermal derating unit TDU, which generate a current as a functionof the temperature of the Light Engine Module LEM. Specifically, in theembodiments shown in FIGS. 4A and 4B, a bipolar transistor is used toamplify the current through a temperature sensitive resistor. However,in general, also other variable current generators could be used.

Moreover, the current generated by the unit TDU could also depend onother parameters. For example, also other sensors could be used to varythe current generated by the unit TDU, e.g.:

-   -   a light sensor configured to detect the ambient luminosity; e.g.        in order vary the light generated by the Light Engine Module LEM        as a function of the ambient light, e.g. in order to keep the        total light quantity substantially constant;    -   a twilight light sensor, e.g. for activating or deactivating the        Light Engine Module LEM as a function of the ambient light;    -   a movement sensor, such as a Passive InfraRed (PIR) sensor, e.g.        for activating or deactivating the Light Engine Module LEM only        in the presence of humans; and/or    -   a wireless receiver for receiving a signal form a remote        control.

The unit TDU may also include a plurality of such sensors in order toimplement different functions. For example, in an embodiment, the unitTDU may include a digital processor, such as a microprocessor, whereinat least one sensor is connected to the processor via an analog todigital converters and the processor generates the output current via aprogrammable current generator. For example, in this way, more complexcontrol functions could be implemented.

Finally, instead of performing a tapping between a portion of theplurality of series connected LEDs, the supply signal for the unit TDUmay also be obtained in a different manner. For example, the supplysignal for the unit TDU may be generated by a current or voltagegenerator connected directly to the supply line LED+. Moreover, inprinciple, the supply signal could also be derived from the voltage Vk.

Thus, the Light Engine Module LEM disclosed herein includes a passivecomponent (i.e. the resistor Rset), which specifies the nominal currentrequirements, and an active current generator, which generates acompensation current, thus modifying the current I_(CL) flowing throughthe control line CL. Specifically, in various embodiments, thiscompensation current is determined as a function of certain operationconditions detected by means of one or more sensors. Due to the factthat the power supply unit PSU varies the supply current for the LightEngine Module LEM as a function of the current I_(CL) flowing throughthe control line CL, the current generator of the unit TDU may modifythe current requirements of the Light Engine Module LEM.

FIG. 5 shows an embodiment of the Power Supply Unit's PSU interface.This is a very simple circuit for cheaper Power Supply Units, where nohigh accuracy is needed.

Due to the requirement of reduced connection lines and the concept of acommon ground line LED−, the problem of voltage drop on that commonGround line LED− due to the Light Engine Module current(s) for the LEDsarises. The embodiment adopts a very simple circuit based on a singleoperational amplifier, without any compensation of the ground lineoffset due to the Light Engine Module current. The Power Supply Unitinterface includes an operational Amplifier OpAmp, where its negativeinput is connected to the communication line CL. The output generates aninternal measurement signal Vout, which is used to adjust the currentTout provided at the output of the Power Supply Unit. The output of thePower Supply Unit is connected to LED+ and LED− of the Light EngineModule. A current measurement resistor Rfb is connected between theoutput and the negative input of the operational amplifier OpAmp, thusforming its mandatory negative feedback. A voltage source Vk isconnected between the positive input of the operational amplifier OpAmpand the common ground line LED−, thus forming the reference for the PSUsinterface.

Actually, just by choosing an adequate value for Vk, the measuring errorcan be reduced until a reasonable value for the application. Forexample, stating a 50 mV max voltage drop on the ground path (1 A on a50 mOhm connection), a 5V voltage is the minimum value for Vk to have anerror due to the voltage drop of under 1%.

To achieve a better accuracy, different compensating techniques for thatcommon ground line offset may be applied. One of the simplest is ofcourse to switch-off the Light Engine Module string before to read outRset: this can be done at the system start-up by a simple machine basedon a sample & hold system.

It must be noticed that when the Light Engine Module string is turnedoff by removing the supply on the LED+ wire, the current level on thecommunication line CL is not affected by the temperature signal. This isnot a disadvantage, because this information is not needed when theLight Engine Modules are completely turned off, rather it is a way toread the Rset value not only with a better accuracy, but also withoutany deviation due to a possible overheating, respective without anydeviation due to the Light Engine Module temperature.

On the other hand, also the opposite way is viable. This means that thepure temperature information is available by simply separating thereference voltage Vk from the OpAmps positive input. Doing so makes thevoltage on the third wire be a function of solely the Light EngineModule temperature (the highest one in case of multiple connection),even in case it's lower than the derating threshold. This makes thePower Supply Unit able to derate itself the current to the Light EngineModule(s), according its proper law, and allows to know the workingtemperature of the Light Engine Module(s) even when not overheated (ofcourse Rset must be known to achieve the best temperature accuracy).

FIG. 6 shows a derating curve of the inventive Power Supply Unit. Thecurve shows the internal control voltage Vout of the Power Supply Unitover the temperature of the Light Engine Module(s). The multiple curvesrelate to the different current requirements of the connected LightEngine Module(s). It can be seen that the derating starts at atemperature of about 93° C. until about 100° C. to 104° C. the power isshutdown completely.

The function of the inventive interface will be explained in thefollowing with the help of a practical example.

As can be seen in the figure, an output current of 1 A results in aninternal measurement signal Vout of 10 V. The interface shall bedesigned in a way, that a conductance of 1 mS for Rset results in anOutput current of 1 A. According to the figure, the voltage source Vk isadjusted to 5 V. This means, that 5V are applied to Rset (see FIG. 5).The operational Amplifier works in a way to minimize the signal Level onits inputs, so it will work until the level at the positive input is thesame like the level at the negative input. So if Vk has 5V, this meansthat 5V will also be at the negative input of the operational amplifier.This leads to 5V at the current set resistor Rset, resulting in acurrent through the communication line CL of 5 V/1 kOhm=5 mA. 5 mAthrough the communication line CL means that these 5 mA also flowthrough the current measurement resistor Rfb, because the input of theoperational amplifier has a high impedance and therefore no currentconsumption As the voltage of the internal measurement signal Vout shallbe 10 V according to FIG. 6, the voltage over the current measurementresistor Rfb has also to be 5 V resulting in a current measurementresistor Rfb with a value of also 1 kOhm respective 1 mS.

According to this example, a Light Engine Module with a currentrequirement of 2 A would have a current set resistor Rset of 2 mS, thatis 500 Ohms.

As mentioned above, the inventive three wire interface with the conceptof the measuring current returning through the common ground linetogether with the LED current has the disadvantage of corrupting themeasuring signal with the voltage drop on the common ground line LED−due to the Light Engine Modules' current flowing through it, but with aproper strategy it is possible to compensate this effect in order toretrieve the true value for the Power Supply Unit.

FIG. 7 shows a schematic circuit of how to model the cable voltage dropVo due to LED current.

The general method to compensate the voltage drop is to vary the Vkvoltage of the voltage source in the Power Supply Unit. The voltage dropcan be cleared out by a linear equation system based on two differentvalues of Vk. Raising the Vk voltage beyond V1 inside the Light EngineModule makes the Rset value uncorrupted by the temperature information(whichever it could be) without turning off the Light Engine Modulepower.

As shown in FIG. 7, the voltage drop on the common return LED− can bemodelled as a voltage generator Vo in series with Rset: The circuit issimilar to the circuit in FIG. 5 with the temperature section left outand added offset generator Vo, which is representing the voltage drop onthe cable.

Now the circuit's equation is formulated by simply considering both OpAmp inputs are at the same voltage:

${\frac{V_{out} - V_{k}}{R_{fb}} = \frac{V_{k} - V_{o}}{R_{set}}},$

or, equivalently,

$\begin{matrix}{{\frac{R_{set}}{R_{fb}}\left( {V_{out} - V_{k}} \right)} = {V_{k} - {V_{o}.}}} & \lbrack 1\rbrack\end{matrix}$

Now, calling

${K_{R} = \frac{R_{set}}{R_{fb}}},$

we can solve [1] into V_(o) (constant), and apply two different valuesfor V_(k):

$\begin{matrix}\left\{ \begin{matrix}{V_{o} = {V_{k,1} - {K_{R}\left( {V_{{out},1} - V_{k,1}} \right)}}} \\{V_{o} = {V_{k,2} - {K_{R}\left( {V_{{out},2} - V_{k,2}} \right)}}}\end{matrix} \right. & \lbrack 2\rbrack\end{matrix}$

It is possible to solve this linear system by equation comparison,finally having:

V _(k,1) −V _(k,2) =K _(R)(V _(out,1) −V _(k,1) −V _(out,2) +V_(k,2))  [3]

This equation can also be written in terms of differencesΔV=V₁−V₂ and solved into K_(R):

$\begin{matrix}{K_{R} = \frac{\Delta \; V_{k}}{{\Delta \; V_{out}} - {\Delta \; V_{k}}}} & \lbrack 4\rbrack\end{matrix}$

This expresses the ratio between the known and the unknown resistors asa ratio of superimposed (Vk) and measured (Vout) voltage differences.As can be seen, the voltage drop V₀ can be computationally eliminated bytwo measurements and some mathematics.

FIG. 8 shows a characteristic of the Current Generator according to theexample of FIG. 6. The graph shows the input of the Current GeneratorCG, the internal measurement signal Vout, against the output current ofthe Current Generator CG Tout. It can be seen, that under a certainvoltage, here called Vsilent, no Output current is provided. At themaximum of the internal measurement signal VoutMax, the maximumspecified output current of the Current Generator CG is provided.Vsilent is the voltage up to where no current flows on the communicationline CG. This can be due to the voltage Vk or due to the TemperatureDerating Unit TDU creating a current I_(TDU) similar to the currentcreated by Vk, but in the opposite direction. So this current creates avoltage over Rset similar to VK, therefore no current flows over thecommunication line CL.

Under normal circumstances, a lighting system would be designed in a waythat no current is provided by the Current Generator CG if no currentflows over the communication line CL. This is because if the conditionof a miswiring or a weak contact exists, no power should be providedfrom the Power Supply Unit PSU to the Light Engine Modules LEM. Butunder certain circumstances, this provision can be amended.

For normal circumstances, if no power should be provided from the PowerSupply Unit PSU to the Light Engine Modules LEM, when no current flowson the communication line CL, the Voltage Vsilent is the same as theVoltage Vk.

FIG. 9 shows a characteristic of the Current Measurement Unit CMU. Amain part of the Current Measurement Unit CMU is the current measurementresistor Rfb. The characteristic shows the output of the CurrentMeasurement Unit CMU, the internal measurement signal Vout, against thenormalized current measurement resistor Rfb/RsetMin. RsetMin is theminimal Value leading to the maximal specified output current IoutMax ofthe Power Supply Unit PSU. So at the value 1, when Rfb=Rsetmin, thePower Supply Unit provides maximal current and Power at its output, andthe internal measurement signal Vout is 2*Vk as described in the exampleof FIG. 6.

In various embodiments of the inventive interface allows to acquire:

-   -   A composite information from the Light Engine Module, i.e. a        nominal current derated by over-temperature, or    -   A split information about nominal current and working        temperature by properly switching the different generators        inside the Power Supply Unit. This of course involves a logic        circuit, and it's not as simple as reading a composite,        non-compensated value from the communication line CL.    -   These are different approaches to read the Light Engine Module        communication line CL, but the electronics inside the module        stays the same.

These and other advantages of the disclosure are summarized in thefollowing:

The inventive interface uses only a simple resistor to set the requiredcurrent.

Only one extra wire is required besides the power connection to theLight Engine Modules.

More Light Engine Modules are allowed to be connected in parallel on thesame bus interface.

The thermal derating can be realized by only adding a simple PTC or fourcheap components.

The auxiliary supply for thermal derating is simply derived from a LightEngine Module string tapping.

The interface is intrinsically fail-safe, in the sense that, if Rset isbroken or the communication line disconnected (the most likely faultevents), the output current is switched off.

In case of short-circuit fault between Light Engine Module+ and thethird wire (could be a wrong connection), the output current isintrinsically switched off, so also preserving the interface circuitryitself.

The Thermal derating unit doesn't drain current from Light EngineModule's supply until Light Engine Module overheating.

The current used to read out Rset can be varied according to the PowerSupply Unit rating, in order to limit its ranging (and improve accuracy)according to the expected applied load.

The inverse Ohm law allows to keep a constant percentage resolution ofoutput current.

The accuracy on reading out Rset depends on the complexity of the PowerSupply Unit side interface, which can be arranged according to expectedsystem quality grade. Furthermore, the reading of Rset may beratiometric to a reference resistor inside the PSU, without requiringaccurate voltage or current sources as in the related art.

The invented interface may provide different information according theapplied stimulus, ranging from a single thermal derated current to twoindependent and accurate values of nominal current and workingtemperature.

While the disclosed embodiments have been particularly shown anddescribed with reference to specific embodiments, it should beunderstood by those skilled in the art that various changes in form anddetail may be made therein without departing from the spirit and scopeof the disclosed embodiments as defined by the appended claims. Thescope of the disclosed embodiments is thus indicated by the appendedclaims and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced.

REFERENCE CHARACTER LIST

-   -   PSU Power Supply Unit    -   LEM Light Engine Module    -   CMU Current Measurement Unit    -   TDU Temperature Derating Unit    -   PGU Pulse Generation Unit    -   PG Pulse Generator    -   CL communication line    -   CG Current Generator    -   Vout internal measurement signal    -   Rset current set resistor    -   Rthr threshold set resistor    -   Rtg emitter resistor    -   Rfb current measurement resistor    -   Rp Pulse Resistor    -   LED+ positive power supply line    -   LED− common ground line    -   V1 voltage source    -   Vk voltage source    -   Vout internal measurement signal    -   S pulse switch

What is claimed is:
 1. A Power Supply Unit comprising: an outputproviding electrical power between a positive power supply line and acommon ground line, a communication line, an adjustable currentgenerator responsive to an internal measurement signal generating anoutput current-at the output, a voltage source coupled to thecommunication line, a current measurement unit, measuring a currentthrough the communication line and generating the internal measurementsignal depending on the measured current through the communication line,wherein said Power Supply Unit is configured to determine the voltagedrop on said common ground line by applying two different voltages tosaid communication line via said voltage source.
 2. The Power SupplyUnit of claim 1, where the current measurement unit comprises a currentmeasurement resistor and an operational amplifier.
 3. The Power SupplyUnit according to claim 2, where: the voltage source is connectedbetween the common ground line and a positive input of the operationalamplifier, the current measurement resistor is connected between anegative input of the operational amplifier and an output of theoperational amplifier, the communication line is connected to thenegative input of the operational amplifier, the output of theoperational amplifier outputs the internal measurement signal.
 4. ThePower Supply Unit according to claim 1, wherein a linear characteristicexists between the internal measurement signal and the output current ofthe adjustable current generator.
 5. The Power Supply Unit according toclaim 4, wherein the linear characteristic is valid only over a portionof the total range of the internal measurement signal.
 6. The PowerSupply Unit according to claim 5, wherein the beginning of the portionis defined by an offset voltage of the internal measurement signal, andbelow the offset voltage no current is provided by the adjustablecurrent generator.
 7. The Power Supply Unit according to claim, whereinthe offset voltage is identical to the value of the voltage source. 8.The Power Supply Unit according to claim 1, wherein the Power SupplyUnit is configured to power at least one Light Engine Module comprising:a plurality of LEDs connected in series between said positive powersupply line and said common ground line, a current set resistor with itsvalue of conductance being indicative for the current demand of theLight Engine Module, wherein said current set resistor is connectedbetween said communication line and said common ground line, and avariable current generator connected to said communication line andgenerating a compensation current, thus modifying the current flowingthrough said communication line, said variable current generator beingresponsive to at least one measurement signal provided by at least onesensor.
 9. The Power Supply Unit according to claim 8, wherein saidPower Supply Unit is configured to read the value of said current setresistor when said output current at said output is turned off.
 10. ThePower Supply Unit according to claim 8, wherein said Power Supply Unitis configured to measure the current generated by said variable currentgenerator of said Light Engine Module when said output voltage of saidvoltage source is deactivated.
 11. The Power Supply Unit according claim8, wherein the voltage source is adjusted in a way that-each LightEngine Module can decrease the supplied current to substantially zero.12. A lighting system, comprising: a Power Supply Unit comprising: anoutput providing electrical power between a positive power supply lineand a common ground line, a communication line an adjustable currentgenerator responsive to an internal measurement signal generating anoutput current at the output, a voltage source coupled to thecommunication line, a current measurement unit, measuring a currentthrough the communication line and generating the internal measurementsignal depending on the measured current through the communication line,wherein said Power Supply Unit is configured to determine the voltagedrop on said common ground line by applying two different voltages tosaid communication line via said voltage source; and at least one LightEngine Module, the Light Engine Module comprising: the positive powersupply line, the common ground line, a plurality of LEDs connected inseries between said positive power supply line and said common groundline, the communication line, a current set resistor connected betweenthe communication line and the common ground line, with its value ofconductance being indicative for the current demand of the Light EngineModule, a variable current generator connected to said communicationline and generating a compensation current, thus modifying the currentflowing through said communication line, said variable current generatorbeing responsive to at least one measurement signal provided by at leastone sensor.
 13. The lighting system according to claim 12, wherein saidvariable current generator comprises: an amplifier responsive to atemperature sensitive input current, a temperature sensitive resistorresponsible for the temperature sensitive input current.
 14. Thelighting system according to claim 13, said Light Engine Module furthercomprising a threshold set resistor responsible for a threshold levelfor thermal derating.
 15. The lighting system according to claim 12,said Light Engine Module comprising: a voltage source with the negativeoutput connected to the common ground line, a series connection of thetemperature sensitive resistor and the threshold set resistor connectedin parallel to the voltage source, a transistor where the base isconnected to the node between the temperature sensitive resistor and thethreshold set resistor, the collector is connected to the node betweenthe positive output of the voltage source and the temperature sensitiveresistor, the emitter is connected to the communication line via anemitter resistor.
 16. The lighting system according to claim 14, saidLight Engine Module comprising: a voltage source with the negativeoutput connected to the common ground, a series connection of thetemperature sensitive resistor and the threshold set resistor connectedin parallel to the voltage source, a transistor where the base isconnected to the node between the temperature sensitive resistor and thethreshold set resistor, the emitter is connected via a emitter resistorto the node between the positive output of the voltage source and thetemperature sensitive resistor, the collector is connected to thecommunication line.
 17. The lighting system according to claim 16, wherethe voltage source is derived from the two power supply lines.
 18. Thelighting system according to claim 16, where the voltage sourceconnected with common ground and tapped between a portion of theplurality of series connected LEDs.
 19. The lighting system according toclaim 12, wherein said at least one sensor comprises: a temperaturesensor configured to detect the temperature of said Lighting EngineModule and/or said plurality of series connected LEDs; a light sensorconfigured to detect the ambient luminosity; a twilight light sensor; amovement sensor; and/or a wireless receiver for receiving a signal froma remote control.
 20. The lighting system according to claim 12, whereineach Light Engine Modules are connected in parallel.